Electronic and vibrational properties of carotenoids: from in vitro to in vivo

Carotenoids are among the most important organic compounds present in Nature and play several essential roles in biology. Their configuration is responsible for their specific photophysical properties, which can be tailored by changes in their molecular structure and in the surrounding environment. In this review, we give a general description of the main electronic and vibrational properties of carotenoids. In the first part, we describe how the electronic and vibrational properties are related to the molecular configuration of carotenoids. We show how modifications to their configuration, as well as the addition of functional groups, can affect the length of the conjugated chain. We describe the concept of effective conjugation length, and its relationship to the S-0 -> S-2 electronic transition, the decay rate of the S-1 energetic level and the frequency of the nu(1) Raman band. We then consider the dependence of these properties on extrinsic parameters such as the polarizability of their environment, and how this information (S-0 -> S-2 electronic transition, nu(1) band position, effective conjugation length and polarizability of the environment) can be represented on a single graph. In the second part of the review, we use a number of specific examples to show that the relationships can be used to disentangle the different mechanisms tuning the functional properties of protein-bound carotenoids.